In order to understand the mechanism of carcinogenesis, it is essential to know the molecular mechanisms involved in regulation of gene expression. Human actin genes were chosen to study the structure, organization and expression of genes. A family of human actin genes has been isolated by molecular cloning. Five of these genes (cardiac muscle actin, aorta-type smooth muscle actin, stomach-type smooth muscle actin, pseudo beta-actin and beta-actin), have been characterized by DNA sequencing. Although there are many pseudogenes for cytoplasmic actin (beta- and gamma-actin), the number of functional genes seems to be only one per haploid genome for each actin isoform. Comparison of the primary structure of the actin genes, particularly the location of introns and the presence or absence of a cysteine codon following the initiation codon, led to a hypothesis for the evolutionary pathways of the actin gene families.